Why interstitial hydrides have a lower density than the parent metal.

Explanation

d-block elements form metallic or interstitial hydrides, on heating with dihydrogen under pressure.
(ii) Hydrogen atom being small in size occupy some in the metallic lattice producing distorting without any change in its type
(iii) The densities of these hydrides are lower then those of metals from which they are formed since the crystal lattice expands due to the inclusion of dihydrogen .